docs/html/training/wearables/data-layer/messages.jd

page.title=Sending and Receiving Messages

@jd:body

<div id="tb-wrapper">
<div id="tb">

<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#SendMessage">Send a Message</a></li>
  <li><a href="#ReceiveMessage">Receive a Message</a></li>
</ol>
</div>
</div>

<p>You send messages using the
<a href="{@docRoot}reference/com/google/android/gms/wearable/MessageApi.html"><code>MessageApi</code></a>
and attach the following items to the message:</p>

<ul>
  <li>An arbitrary payload (optional)</li>
  <li>A path that uniquely identifies the message's action</li>
</ul>
<p>
Unlike data items, there is no syncing between the handheld and wearable apps.
Messages are a one-way communication mechanism that's good for remote procedure calls (RPC),
such as sending a message to the wearable
to start an activity. You can also use messages in request/response model
where one side of the connection sends a message, does some work,
sends back a response message.</p>

<h2 id="SendMessage">Send a Message</h2>

<p>The following example shows how to send a message that indicates to the other
side of the connect to start an activity.
This call is made synchronously, which blocks until the message
is received or when the request times out:
</p>

<p class="note"><b>Note:</b> Read more about asynchronous and synchronous calls
to Google Play services and when to use each in
<a href="{@docRoot}google/auth/api-client.html#Communicating">Communicate with Google Play Services</a>.
</p>

<pre>
Node node; // the connected device to send the message to
GoogleApiClient mGoogleApiClient;
public static final START_ACTIVITY_PATH = "/start/MainActivity";
...

    SendMessageResult result = Wearable.MessageApi.sendMessage(
            mGoogleApiClient, node, START_ACTIVITY_PATH, null).await();
    if (!result.getStatus().isSuccess()) {
        Log.e(TAG, "ERROR: failed to send Message: " + result.getStatus());
    }
</pre>

<p>
Here's a simple way to get a list of connected nodes that you can potentially
send messages to:</p>

<pre>
private Collection&lt;String&gt; getNodes() {
    HashSet &lt;String&gt;results= new HashSet&lt;String&gt;();
    NodeApi.GetConnectedNodesResult nodes =
            Wearable.NodeApi.getConnectedNodes(mGoogleApiClient).await();
    for (Node node : nodes.getNodes()) {
        results.add(node.getId());
    }
    return results;
}
</pre>

<h2 id="ReceiveMessage">Receiving a Message</h2>

<p>

To be notified of received messages, you implement a listener for message events.
This example shows how you might do this by checking the <code>START_ACTIVITY_PATH</code>
that the previous example used to send the message. If this condition is <code>true</code>,
a specific activity is started.
</p>

<pre>
&#64;Override
public void onMessageReceived(MessageEvent messageEvent) {
    if (messageEvent.getPath().equals(START_ACTIVITY_PATH)) {
        Intent startIntent = new Intent(this, MainActivity.class);
        startIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        startActivity(startIntent);
    }
}
</pre>

<p>
This is just a snippet that requires more implementation details. Learn about
how to implement a full listener service or activity in
<a href="{@docRoot}training/wearables/data-layer/events.html">Listening for Data Layer Events</a>.
</p>